1. Field of the Invention
The present invention relates to a paper thickness detecting apparatus using an electrostatic sensor device.
2. Description of the Prior Art
In a copying machine or a bill identification system, various troubles occur if two sheets of paper are fed. For this reason, these systems include a paper thickness detecting apparatus. When two sheets of paper are fed, the apparatus detects this and stops the system, or the two sheets of paper are discharged by discharging means.
FIG. 1 shows a general paper thickness detecting apparatus used in a copying machine or a bill identification system as described above. In this apparatus, a potentiometer 3 is arranged on a path 2 of paper 1. This potentiometer 3 includes a rotating shaft 3a and a probe 3b mounted on the rotating shaft 3a, and the distal end of the probe 3b is constantly biased against the paper 1 so as to be in contact with the paper 1. The probe 3b is rotated in a direction indicated by an arrow as a paper thickness changes, and a rotational angle of the probe 3b is converted into a voltage change and extracted.
In a conventional apparatus using a potentiometer, however, a paper thickness is detected while the probe 3b is in contact with the paper 1. When a running speed of paper is increased, therefore, the movement of the probe 3b cannot follow the movement of the paper. Therefore, the running speed of paper is limited.
In addition, since a change in thin paper such as a copying sheet or bill is detected by the rotational angle of the probe 3b, a change in rotational angle of the probe 3b with a change in paper thickness obtained when two sheets of paper are fed is very small. In order to correctly detect a change in paper thickness, therefore, the potentiometer 3 must have very high precision. As a result, the arrangement of the apparatus is complicated, and the manufacturing cost of the apparatus is increased.